The life of hot cathode fluorescent lamps is primarily limited by wearing of filaments. Most often, lamp failure is preceded and caused by filament failure wherein an erosion of filament material results in filament failure and, consequently, lamp failure. Moreover, hot cathode fluorescent lamps usually fail in an abrupt fashion. Namely, as the lamp ages and its filament wears, there are usually no obvious early signs of approaching failure.
In typical fluorescent lamps, filaments are electrodes positioned at opposite ends of the lamp tube. The negative electrode, or cathode, emits electrons into the gas discharge while the positive electrode, or anode, collects the electrons at the other end of the discharge. In typical applications, the lamp operates in an AC mode such that the filaments alternate between functioning as cathode and anode during each power cycle. Filaments are typically made of coiled tungsten wire coated with emission mix material which has a low work function, i.e., emits electrons easily when heated to a high temperature. For proper lamp operation and to maximize lamp life, it is necessary to limit erosion of filament material by maintaining filament temperature in a relatively narrow range. If the filament is allowed to operate below or above this temperature range, its life will be adversely affected. For example, if the filament temperature is lower than optimum, the number of thermionic electrons is insufficient to maintain desired discharge current, causing the voltage between the cathode and the discharge, i.e. the cathode fall voltage, to increase. As a result, ions collected by the cathode get accelerated as they reach the cathode, impacting the electrode surface at a higher speed and causing additional electrons to be emitted. The process is called ion bombardment and leads to sputtering of filament material and shorter lamp life. Conversely, if the filament temperature is allowed to increase too much, evaporation of emission material from electrodes will take place, accelerating filament wear and shortening lamp life.
There exist several methods for maintaining filament temperature in the optimum range. For lamps operated at constant current, lamp filaments may be specifically designed for that current, so that heat generated by the discharge current maintains the filament temperature close to optimal. Another approach is to add a shunt capacitance in parallel with the lamp at the output of a high frequency electronic ballast. By connecting a shunt capacitance in series with two filaments, part of the ballast output current is diverted through the filaments, generating heat. Capacitor size is optimized for required filament heating. In dimming applications wherein lamp current varies over a wide range depending on selected light output, a separate filament heater power supply may be used to adjust filament heating depending on the lamp current, such as described in commonly assigned U.S. Pat. No. 5,703,441 of R. L. Steigerwald, Chester F. Saj, and Ljubisa D. Stevanovic, issued Dec. 30, 1997.
Unfortunately, even if filament temperature is optimal, erosion of filament material continually takes place through processes of evaporation and sputtering, eventually causing filaments to break. After a filament breaks, the lamp usually continues to run for a relatively short period of time before it fails, e.g., several hours to several weeks. The length of this "residual life" depends on a number of factors, such as output voltage capability of the ballast, whether there is a dimming function, inter alia. Filament breakage also disconnects circuitry provided for additional heating, such as the hereinabove described shunt capacitor or filament heater supply, causing filament temperature to drop and thus further accelerating the process of filament erosion through ion bombardment until the emission mix sputters away completely. At that point, tungsten and copper in the filament wire and contacts are the only sources of electrons. Both of these materials have a relatively high work function and thus will start to heat up and increase the temperature of the lamp base, causing the glass tube to crack and, in turn, causing leakage of the gas fill, resulting in lamp failure.
Hence, by maintaining filament temperature close to optimal, lamp life can be extended significantly. Nowadays, commercially available hot cathode fluorescent lamps have rated life up to twenty thousand hours. This is more than adequate for most applications, such as household and office lighting. However, in high reliability, high performance systems, such as backlighting for aircraft LCD, lamp failure is the predominant failure mode and is a definite limitation. The problem is further exacerbated by the unpredictability of the failure mechanism of the lamp, as described hereinabove.
Accordingly, it is desirable to provide a simple and reliable diagnostic circuit and method for detecting filament failure, i.e., breakage, and thus indicating that the lamp needs to be replaced before it actually fails. Further, since the voltage difference between two filaments of one lamp can be as high as several hundred volts during starting, the diagnostic circuit should be isolated from at least one filament. Still further, it is desirable that such diagnostic circuit not interfere with normal operation of the lamp, ballast, and filament heater power supply.